Inhibitory and excitatory neurons exhibit distinct patterns of development in the mammalian cerebral cortex. The morphological development of inhibitory and excitatory neurons derived from fetal rat cerebral cortex has now been compared in vitro. Inhibitory neurons were identified by immunofluorescence staining with antibodies to γ-arninobutyric acid, and axon formation was detected by staining with antibodies to phosphorylated neurofilaments. In chemically defined, gliafree, and low-density cultures, excitatory neurons formed axons within 3 days of plating. In contrast, inhibitory neurons required more than 6 days to form axons. Time-lapse analysis over 6 days revealed that most inhibitory neurons were bipolar and that their two processes exhibited alternate growth and retraction without giving rise to axons. Movement of the cell body toward the growing process was apparent in about one-half of inhibitory neurons, whereas such movement was never seen in excitatory neurons. The migratory behavior of neurons was further investigated by culture on a glial cell monolayer. Inhibitory neurons migrated over substantially larger distances than did excitatory neurons. The centrosome of inhibitory neurons translocated to the base of the newly emerging leading process, suggesting the existence of a force that pulls intracellular organelles toward the leading process. Centrosome translocation was not detected in excitatory neurons. These observations suggest that the developmental programs of excitatory and inhibitory neurons differ : Inhibitory neurons thus possess a more effective cytoskeletal machinery for migration and form axons later than do excitatory neurons.